Skin is the first barrier to environmental exposures such as ultraviolet radiation, which is divided into UVC radiation (200-280 nm), UVB radiation (280-320 nm) and UVA radiation (320-400 nm). Compared to UVC radiation, which is predominately filtered by the ozone layer, UVB radiation is mainly absorbed by the epidermis and can induce skin cancers in animals due to DNA damage (Hussein. J Cutan Pathol, 2005. 32: p. 15; herein incorporated by reference in its entirety). UVA radiation can penetrate through dermis to subcutaneous tissues and induce various biological responses, ranging from erythema to photoaging (Zanatta et al. Food and Chemical Toxicology, 2010. 48(1): p. 70-75; herein incorporated by reference in its entirety). It is now recognized that both UVA and UVB wavelengths, received during daily exposure to sun, contribute to chronic photodamage of human skin (Gonzalez et al. Clin Dermatol, 2008. 26(6): p. 614-26; herein incorporated by reference in its entirety). Adequate protection against solar UV exposure can prevent cellular and molecular changes that lead to photoaging, photoimmunosuppression and photocarcinogenesis (Seite. Photodermatology Photoimmunology Photomedicine, 2000. 16; herein incorporated by reference in its entirety).
There are 17 active sunscreen ingredients currently approved by the FDA, including various organic and inorganic compounds that are used as filters to help prevent skin damage. Some of these compounds may have adverse effects such as contact sensitivity, increased risk of vitamin D deficiency, and estrogenicity (Sambandan & Ratner. J Am Acad Dermatol, 2011. 64(4): p. 748-58; herein incorporated by reference in its entirety). Organic chemicals, such as avobenzone, are strong absorbers; whereas, inorganic compounds, such as zinc or titanium oxides, use scattering and absorption mechanisms for protection. Although zinc oxide is the most efficient at protecting against both UVA and UVB radiation, products that use this ingredient leave an undesirable opaque and often dense film on the skin, making their proper use uncomfortable due to increased retained body heat and cosmetically unpleasing. This problem is more prominent for people with darker shades of skin color. Although nanoparticle versions of the metal oxides appear to have better skin application and spreading properties, they only marginally improve the aesthetically unpleasing effect when used on skin and the safety and fate of the nanoparticles in the human body remain a concern (Sambandan & Ratner. J Am Acad Dermatol, 2011. 64(4): p. 748-58; herein incorporated by reference in its entirety). The use of avobenzone eliminates the consistency and aesthetic problems encountered with metal oxides, but sunscreen formulations that use it require chemical photostabilizers in order for the protective absorption spectra to be maintained for a 2 to 3-hour duration (Gonzalez et al. Clin Dermatol, 2008. 26(6): p. 614-26; herein incorporated by reference in its entirety). The efficacy of avobenzone is also very sensitive to the ingredients used in the sunscreen formulation (Beasley. Am J Clin Dermatol, 2010. 11(6): p. 10; herein incorporated by reference in its entirety). As for other small molecular UV absorbers such as aminobenzoates and benzophenones for topical application, their safety remains a concern (Gonzalez et al. Clin Dermatol, 2008. 26(6): p. 614-26; herein incorporated by reference in its entirety). Hence, there is a need for new materials that protect from UVA and UVB radiation that are safe, effective, inexpensive, and can be easily formulated for a skin application that is aesthetically pleasing and therefore would be used by many more people to help curb the rising rates of skin cancer.